Narrow band excimer laser with gas additive

ABSTRACT

The present invention provides a very narrow band pulse excimer laser capable of producing pulses at a rate in the range of about 500 to 2000 Hz with enhanced energy dose control and reproducibility. Very small quantities of a stablizing additive consisting of oxygen or a heavy noble gas (xenon or radon for KrF lasers, or krypton, xenon or radon for ArF lasers), are added to the gas mixture. Tests performed show substantial improvements in energy stability with the addition of about 30 ppm of xenon to a KrF laser. Tests show improved performance for the ArF lasers with the addition of about 6-10 ppm of Xe or 40 ppm of Kr. In a preferred embodiment very narrow bandwidth is achieved on a KrF laser by reducing fluorine partial pressure to less than 0.10 percent and by increasing the reflectance of the output coupler to greater than 25 percent. In a preferred embodiment, prior art fused silica beam expansion prisms used in the prior art line-narrowing module were replaced with calcium fluoride prisms.

This is a Continuation-In-Part application of Ser. No. 08/947,474, VeryNarrow Band KrF Laser, filed Oct. 10, 1997. This invention relates tolasers and in particular to narrow band lasers.

BACKGROUND OF THE INVENTION

Excimer lasers are currently becoming the workhorse light source for theintegrated circuit lithography industry. A typical prior art KrF excimerlaser is depicted in FIG. 1 and FIG. 9. A pulse power module AE provideselectrical pulses lasting about 100 ns to electrodes 6 located in adischarge chamber 8. The electrodes are about 28 inches long and arespaced apart about 3/5 inch. Typical lithography lasers operated at ahigh pulse rate of about 1,000 Hz. For this reason it is necessary tocirculate a laser gas (about 0.1 percent fluorine, 1.3 percent kryptonand the rest neon which functions as a buffer gas) through the spacebetween the electrodes. This is done with tangential blower 10 locatedin the laser discharge chamber. The laser gasses are cooled with a heatexchanger also located in the chamber. Commercial excimer laser systemsare typically comprised of several modules that may be replaced quicklywithout disturbing the rest of the system. Principal modules are shownin FIG. 2 and include:

Laser Chamber 8,

Pulse Power Module 2,

Output coupler 16,

Line Narrowing Module 18

Wavemeter 20

Computer Control Unit 22

Peripheral Support Sub systems

Blower 10

The discharge chamber is operated at a pressure of about threeatmospheres. These lasers operate in a pulse mode at about 600 Hz toabout 1,000 Hz, the energy per pulse being about 10 mJ and the durationof the laser pulses is about 15 ns. Thus, the average power of the laserbeam is about 6 to 10 Watts and the average power of the pulses is inthe range of about 700 KW. A typical mode of operation is referred to asthe "burst mode" of operation. In this mode, the laser produces "bursts"of about 50 to 150 pulses at the rate of 1,000 pulses per second. Thus,the duration of the burst is about 50 to 150 milliseconds. Prior artlithograph, excimer lasers are equipped with a feedback voltage controlcircuit which measures output pulse energy and automatically adjusts thedischarge voltage to maintain a desired (usually constant) output pulseenergy. It is very important that the output pulse energy be accuratelycontrolled to the desired level.

It is well known that at wavelengths below 300 nm there is only onesuitable optical material available for building the stepper lens usedfor chip lithography. This material is fused silica. An all fused silicastepper lens will have no chromatic correction capability. The KrFexcimer laser has a natural bandwidth of approximately 300 pm (fullwidth half maximum). For a refractive system (with NA>0.5)--either astepper or a scanner--this bandwidth has to be reduced to below 1 pm.Current prior art commercially available laser systems can provide KrFlaser beams at a nominal wavelength of about 248 nm with a bandwidth ofabout 0.8 pm (0.0008 nm). Wavelength stability on the best commerciallasers is about 0.25 pm. With these parameters stepper makers canprovide stepper equipment to provide integrated circuit resolutions ofabout 0.3 microns. To improve resolution a narrower bandwidth isrequired. For example, a reduction of a bandwidth to below 0.6 pm wouldpermit improvement of the resolution to below 0.25 microns.

Argon fluoride, ArF excimer lasers which operate at a wavelength ofabout 190 nm using a gas mixture of about 0.08 to 0.12% fluorine, 3.5%argon and the rest neon, are beginning to be used for integrated circuitlithography.

It is known that the addition of about 10 to 50 ppm of oxygen to anexcimer laser gas mixture can be used to stabilize the efficiency andperformance of the laser. See, for example, U.S. Pat. No. 5,307,364.

The actual performance of integrated circuit lithography equipment thendepends critically on maintaining minimum bandwidth of the laserthroughout its operational lifetime.

Therefore, a need exists for a reliable, production quality excimerlaser system, capable of long-term factory operation and havingaccurately controlled pulse energy stability, wavelength, and abandwidth.

SUMMARY OF THE INVENTION

The present invention provides a very narrow band pulse excimer lasercapable of producing pulses at a rate in the range of about 500 to 2000Hz with enhanced energy dose control and reproducibility. Very smallquantities of a stablizing additive consisting of oxygen or a heavynoble gas (xenon or radon for KrF lasers, or krypton, xenon or radon forArF lasers), are added to the gas mixture. Tests performed showsubstantial improvements in energy stability with the addition of about30 ppm of xenon to a KrF laser. Tests show improved performance for theArF lasers with the addition of about 6-10 ppm of Xe or 40 ppm of Kr. Ina preferred embodiment very narrow bandwidth is achieved on a KrF laserby reducing fluorine partial pressure to less than 0.10 percent and byincreasing the reflectance of the output coupler to greater than 25percent. In a preferred embodiment, prior art fused silica beamexpansion prisms used in the prior art line-narrowing module werereplaced with calcium fluoride prisms.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the principal elements of a prior artcommercial KrF excimer lasers used for integrated circuit lithography.

FIG. 2 is a simplified electrical drawing of a solid state pulse powercircuit.

FIG. 3 are graphs comparing the results of a solid state pulse powercircuit to a prior art thyratron based circuit.

FIG. 4 is a graph of operating voltage during a pulse.

FIG. 5 shows a typical variation of operating voltage and bandwidth overan 800 million pulse period.

FIG. 6 is a simplified sketch of a KrF laser system.

FIG. 7 is a sketch of the principal element of a line-narrowing module.

FIGS. 8A-8J show results of an additive of Xe to a :KrF laser.

FIG. 9 is a drawing of a prior art commercial KrF lithography laser.

FIG. 10 shows the relationship between fluorine, operating voltage andpulse energy.

FIG. 11 shows the variation of line width with fluorine concentrations.

FIGS. 12A and 12B show pulse shape with different fluorineconcentrations.

FIG. 13 is a chart of average pulse energy during the first 125 pulsesduring burst mode operation with no oxygen in the chamber where datafrom 50 bursts were averaged.

FIG. 14 is a chart similar to FIG. 13 showing average pulse energy withoxygen at 0 ppm, 25 ppm and 49 ppm.

FIG. 15 is a chart showing 3-sigma statistics of the data plotted inFIG. 14.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described below.

DESCRIPTION OF LASER

FIG. 1 shows the principal elements of a commercial excimer laser systemof the type used today in integrated circuit lithography.

The Chamber

The discharge chamber 10 is a vessel, designed to hold severalatmospheres of corrosive gases. These vessels are designed to knownsafety standards, such as those specified by ASME. The two electrodesseparated by a gap of 1.2 to 2.5 cm define the discharge region. Thecathode is supported by an insulating structure since it is connected tothe high voltage, while the anode is attached to the metal chamber as itis at ground potential. Preionization is done by corona dischargepreionizers located on either side of the discharge region. Due to thecorrosive nature of the gas the chambers use particular metals chosen toresist fluorine attack. The fluorine gas however, still reacts with thechamber internal parts such as chamber walls and electrodes; thusconsuming fluorine and generating metal fluoride contaminants.

Since the laser is pulsed (500 to 2000 Hz), it is essential to clear thedischarge region between pulses--a task preferably performed by atangential blower, which is magnetically coupled to an external drivesource. Heat is extracted from the laser gas by means of a water-cooledfinned heat exchanger inside the chamber. Metal fluoride dust is trappedby means of an electrostatic precipitator not shown. A small amount oflaser gas is extracted from the chamber and is passed over negativelycharged high field wires to trap the dust. The dust-free gas is thenreleased over the windows to keep them clean. The gas is driven throughthe precipitator by the differential pressure built up inside the laserchamber due to the high velocity flow.

Pulse Power Module

This preferred embodiment utilizes a solid state pulsed power module(SSPPM) circuit shown in FIG. 2. The 20 KV power supply of prior artthyratron systems is replaced by a 1 kV supply. The thyratron switch isreplaced by an SCR switch that does not feed C_(p) directly, but insteadswitches the energy of C₀ into a pulse compression circuit formed by C₁,C₂, C₃, a step-up transformer, and three saturable inductors. Theoperation of this circuit is as follows. The DC charge stored on C₀ isswitched through the SCR and the inductor L₀ into C₁. The saturableinductor, L₁, holds off the voltage on C₁ for approximately 2.5 ns andthen becomes conducting, allowing the transfer of charge from C₁ to C₂.The second saturable inductor, L₂, holds off the voltage on C₂ forapproximately 500 ns and then allows the charge on C₂ to flow throughthe primary of 1:20 step-up transformer. The output from the step-uptransformer is stored on C₃ until the saturable inductor L₃ becomesconducting in approximately 100-150 ns. The charge is then finallytransferred through L₃ into C_(p) and laser discharge occurs.

Spectral Narrowing

As stated earlier, bandwidth (FWHM) of a free running KrF excimer laseris approximately 300 pm. Currently, excimer steppers utilize lasersspectrally narrowed to between 0.8 and 3 pm, FWHM, depending on the NAof the lens. It should be noted that the integrated energy spectrum andthe spectral width at 95% energy are more critical to stepperperformance than the FWHM value. However, most users find it convenientto talk about FWHM instead of spectral width at 95% energy.

Spectral narrowing of a KrF laser is complicated by its short pulseduration (10 to 15 ns, FWHM) and UV wavelength. The short pulse resultsin very high intra-cavity power (˜1 MW/cm²), and the short wavelengthcan thermally distort optical materials due to their high absorptionco-efficient at 248 nm. Also, the total number of round trips throughthe resonator (which includes the line narrowing optical elements) for atypical laser is small, about 3 to 4. If the single pass linewidththrough the resonator is denoted by )Δλ₁, then the final linewidth)Δλ_(f) after n passes is given by: ##EQU1##

Therefore, the single pass linewidth of the optical system should be, atmost, a factor of two higher than the final linewidth. In fact, timeresolved spectral measurements by Applicants fellow workers indicatethat the spectral linewidth could decrease by a factor of two from thestart of the pulse to the tail of the pulse. Therefore, the efficiencyof converting the broadband spectrum to line narrowed spectrum (i.e.from 300 pm to <1 pm) of the optical system must be very high.

The common technique of line-narrowing the KrF laser is by introducingwavelength dispersive optical elements in the resonator. Three types ofdispersive elements can be used: prisms, etalons and gratings. The useof a high dispersive grating in a Littrow configuration is the simplestand most effective spectral line narrowing technique. Because thegrating is a dispersive element, the line-width is proportional to thebeam divergence. To get narrow line-width, a small beam divergence isrequired. Hence, 2 slits and 3 prisms beam expanders are inserted in thelaser resonator. The principal elements of a preferred line-narrowingmodule are shown in FIG. 7. These include 3 prisms; 30, 32 and 34, atuning mirror, 3C and an eschelle grating, 38. The mirror is pivoted tochange the wavelength of the laser.

Improved Spectral Performance

Applicants and their fellow workers have designed, built and testedlaser KrF excimer laser equipment capable of meeting linewidthspecifications of 0.50 pm at FWHM with 95% of the energy of the laserbeam within 2 pm. These results have been demonstrated on new, mid-ageand old discharge chambers for 80 million pulses proving that the systemis capable of continuous performance within these specifications overthe normal life of the equipment with usual maintenance. These resultsrepresent an approximately 50% improvement over the prior art narrowband excimer laser technology.

In order to achieve this improved performance Applicants have improvedboth the laser equipment and the operating parameters of the laser.

Reduction of Fluorine Consumption

In preferred embodiments of the present invention which have been builtand tested by Applicants, great care was taken to eliminate materialsfrom the discharge chamber that consume fluorine. Fluorine consumptionin a discharge chamber is due to fluorine reaction with materials in thechamber. These reactions typically produce contaminants, which result indeterioration of laser performance. In order to minimize fluorineconsumption, this preferred embodiment include the following specificfeatures:

The chamber walls are aluminum coated with nickel.

The electrodes are brass.

All metal O-rings are used as seals.

Insulators are all ceramic and fluorine compatible.

Alumina is applicants preferred insulator material.

An electrostatic filter is provided as in prior art designs to filtercontaminants produced during operation.

The fan unit is driven using a magnetically coupled motor locatedoutside the sealed discharge chamber using a prior art technique.

During manufacture, parts are precision cleaned to remove potentialcontaminants.

After assembly, the chamber is passivated with fluorine.

Reduction of Nominal Fluorine Concentration

This preferred embodiment requires substantial changes in operatingprocedures and parameters of the laser system in order to achieve thedesired very narrowband output. The fluorine concentration in reducedfrom 0.1% (30 kPa) to about 0.06% (18 kPa). The total gas pressure isabout 300 kPa. (The Kr concentration is maintained at the prior artlevel of about 1.3% and the remainder of the laser gas is neon.) Duringoperation, fluorine will be gradually depleted. Constant pulse energy isobtained by gradually increasing the laser operating voltage inaccordance with prior art techniques. Injections of a mixture offluorine and neon are made periodically (typically at intervals of about1 to 4 hours) to make up for the depletion of fluorine in accordancewith techniques well known in the excimer laser prior art. During thisprocedure, the fluorine concentration is preferably maintained withinthe range of between about 0.055% and 0.065% and the operating voltageis maintained within a corresponding range appropriate to maintainconstant pulse energy. For example, in a preferred embodiment this rangewas 670 Volts to 790 Volts.

Increase in Reflectivity of Output Coupler

In this preferred embodiment of the present invention the reflectivityof the output coupler has been increased from about 10% which wastypical of prior art narrow band excimer lasers to about 30%. This wasdone to help make up for the loss of laser efficiency resulting from thereduced fluorine concentration.

Switch to Calcium Fluoride Prisms

The change in the reflectivity of the output coupler from 10% to 30% hadthe effect of approximately doubling the light passing through theline-narrowing module. The additional heat generated by this additionalillumination in the prior art fused silica prisms caused thermaldistortion in the prisms. To solve this problem the fused silica prismswere replaced with calcium fluoride prisms. Calcium fluoride has higherthermal conductivity and could handle the additional energy withoutunacceptable distortion.

Fluorine Reduction

FIG. 10 describes the relationship between operating voltage, fluorineconcentration and pulse energy. This graph shows that as the fluorineconcentration decreases the voltage must be increased to maintain thedesired output of 10 mJ per pulse. However, in this particularembodiment the upper limit on the operating voltage is 800 Volts. Notethat with a 10% R output coupler the lowest fluorine concentrationcorresponding to an output of 10 mJ would be 25 kPa at which point theoperating voltage would have risen to 800 Volts. However with a 30% Routput coupler, the fluorine concentration could be reduced to as low asabout 20 kPa while still maintaining a 10 mJ pulse energy with theoperating voltage at slightly under 800 Volts. FIG. 11 shows actual testresults of reducing the fluorine concentration on line width (measuredat FWHM and at 95% pulse energy) for both continuous pulses at 1000 Hzand for 500 pulse bursts and 1000 Hz. For this particular test theoutput coupler had a 25% reflectivity. Typical laser pulse shapes forprior art KrF systems and these very narrowband KrF lasers are comparedin FIGS. 12A and 12B. Note that with the very narrowband lasers, energyis shifted to the latter part of the pulse, which represents photons,which have had the benefit of more trips through the line-narrowingmodule. As a result, the integrated pulse spectral linewidth of thelaser is reduced.

BURST MODE OPERATION

As indicated in the Background section of this specification, a typicalmode of operation of a KrF laser is a "burst mode" in which bursts ofabout 125 pulses are produced at the rate of 1000 pulses per second. Theburst lasts for about 125 milliseconds and there typically is a "deadtime" of a fraction of a second between bursts. Applicants' KrF lasercontains about 0.017 cubic meters of laser gas and the flow rate of gasbetween the electrodes produced by blower 10 is about 0.228 cubic metersper second. This would imply a total gas circulation time of about 75milliseconds; however, flow in the chamber is far from uniform andportions of the gas circulates much quicker. The speed of the gasbetween the electrodes is about 20 meters per second and the Applicantsestimate that the fastest gas makes the trip in about 20 milliseconds.Applicants have discovered a "slug effect" generated by the first or thefirst few pulses in a burst. This slug effect is shown in FIG. 13 thatis a plot of pulse energy for each of the 123 pulses of a typical burstof 123 pulses averaged over 50 bursts. There is a large drop-off afterthe first pulse and another large dip after the 21st pulse, i.e. about21 milliseconds following the first pulse. This dip is extremelyreproducible and the timing of the dip is in proportion to the fanspeed. Applicants do not know the exact cause of this first 40 secondsof very reproducible erratic performance but have identified it as the"slug effect" and believe it is attributable to chemical effectsgenerated when "clean" laser gas passing between the electrodes isblasted with 20,000 volts during the first pulse or the first fewpulses. The gas passing between the electrodes during the first 30milliseconds is substantially all "clean" laser gas but after about 20milliseconds, gas electrocuted during the first pulse begins to passback between the electrodes. After about 39 milliseconds into the burst,the gas in the laser is thoroughly mixed and the slug effect disappears.

GAS ADDITIVES

Applicants, through their experimentation have discovered thatsubstantial improvements in laser performance can be realized by theaddition of very small quantities of selected gases. Prior art teasesthat about 10 to 50 ppm oxygen improves energy stability. However, thesequantities of oxygen produce a decrease in the power output which tendsto outweigh the improvements in stability. Applicants have discoveredthat smaller quantities of oxygen provide significantly improvedstability without significant detrimental effects. Applicants have alsodiscovered that the addition of very small quantities of heavy noblegases provides substantial improvements without significant detrimentaleffects.

Xenon Additive

The effect of xenon additives on operating voltage and efficiency aregiven in FIG. 8A. The rate of efficiency decrease is about 0.15% per 1ppm of xenon. Energy stability was noticeably improved for all xenonconcentrations, but exhibited a slight maximum around 30 ppm. Thismaximum is not apparent from the drawing. All subsequent tests wereperformed with a xenon concentration of 30 ppm.

The energy versus voltage characteristic is given in FIG. 8B. The energyis lower with xenon over the entire range.

The burst transients are compared in FIGS. 8C and 8D. With xenon theenergy transient is reduced, especially for the first ten pulses, whichmakes it easier on the energy algorithm. A major improvement with xenonis found in the energy stability, which is reduced, for all pulsenumbers. This is in contrast to the effect of oxygen which only works onthe reentrant. In fact, this chamber does not exhibit any reentrant, sothe reentrant effect of xenon could not be confirmed with this chamber.For a blower speed of 4200 rpm the reentrant should occur at about 20ms. (Note, subsequent tests with chambers that do exhibit a reentranteffect confirm that 30 ppm xenon does produce at least a small reductionin the reentrant effect.

The laser energy is almost independent of repetition rate (see FIG. 8E),with the xenon mixture giving consistently lower values. By contrast,the improvement in dose stability with xenon is most noticeable athigher pulse rates. At 1 kHz the energy stability is probably dominatedby effects unrelated to discharge stability such as noise in the dataacquisition and high voltage power supply regulation. We are using twoparalleled 5000 power supplies with >3V of dither. The dose stability ina 2 kHz mode is displayed in FIG. 8F. Addition of 30 pm of xenon reducesthe dose error by about 0.1%. This is a substantial improvement.

No effect of xenon on any other beam parameters (spatial profiles anddivergence, linewidth) was observed. Occasionally, it appeared thatxenon mixtures produce narrower linewidth. However, this was most likelyan artifact produced by the fused silica beam expanding prisms. It takesmore time to generate a xenon mixture, which allows the prisms to cooldown. Recorded linewidths were 0.65 pm FWHM and 1.90 pm 95%. Thelinewidths would possibly be narrower with CaF₂ prisms because of betterthermal properties. A comparison of the temporal profiles at 10 mJ ofenergy is displayed in FIG. 8G. The 30 ppm xenon mixture exhibits atypical waveform for higher charging voltages (667V 30 ppm Xe, 651V w/oXe), namely larger initial spike and shorter duration. From this, onewould expect a larger linewidth with xenon, which was not observed.Nevertheless, the differences are very small and only reflect oneparticular shot. Unfortunately, no averaged pulse profiles wererecorded.

Explanation of Results of Xenon Tests

So why does xenon help at all and why in such small concentration? Someinsight is gained by observing the peaking capacitor voltage Vcp (FIG.8H). For the same charging voltage Vco of 650V, gas breakdown occurs 2ns earlier with a xenon mixture. The obvious explanation is improvedpre-ionization. Xenon can be ionized by light shorter than 93 nm whereaskrypton and neon have thresholds of 85 nm and 58 nm, respectively (R. S.Taylor, IEEE JQE v.31, p.2195, 1995). Therefore, xenon can use a largepart of the corona light that otherwise would just be transmitted. Evenat 30 ppm the xenon concentration is seven orders of magnitude largerthan typical pre-ionization electron densities. This means, the amountof xenon atoms is not a limiting factor. The absorption cross section ofxenon is 1500 cm⁻¹ which translates into a 50% transmission after 5 cmfor 30 ppm at 315 kPa. This would explain why higher xenonconcentrations are less efficient, the 90 nm light is already beingfiltered out very close to the PI tube.

There are other scenarios possible like faster current avalanching dueto the lower ionization potential of xenon. This, however, is hinderedby the low concentration. Another possibility is a change in thespectral content of the corona light, which may have beneficial effects.In fact, the discharge containing xenon visually appears much brighter,primarily due to yellow components.

Better pre-ionization may also help the minimum clearing ratio (ameasure of gas flow between electrodes between pulses). There is a veryslight improvement for lower charging voltages (FIG. 8I). At 650V (10mJ) 3800 rpm is barely enough to prevent down-stream-arcing and dosestability improves when going to 4200 rpm. At 800V arcing is much moresevere, although largely aggravated by blips.

Xenon Effects Survives Refills

Very early in the experiments a strange phenomenon was encountered: thebeneficial effect of xenon survives refills. Due to this, detailedstudies on the influence of the exact xenon concentration becamedifficult, or at least time consuming. What is happening is that afterthe laser was operated with a xenon containing mixture and refilledwithout xenon, the energy stability would stay at a low level. Not asgood as with xenon, but somewhere in between. A number of experimentswere conducted to help understand the mechanism of this memory effect.The difference in dose stability between a truly xenon-free mixture anda pre-conditioned mixture is only 0.05%. This difference is too small toallow any hard conclusions, so only some general trends can be outlined.

There are two possibilities; either xenon physically stays in thechamber or it altars the chamber in a long lasting way. Such analteration could be smoothing or cleaning of electrodes or windows. Afirst refill after a xenon fill was operated for four hours and 2million shots without losing good stability. Four to five refills,however, with much fewer shots and in a shorter time completely bringthe chamber back to normal. This rather supports the theory that xenonstays in the chamber. The same conclusion is drawn from the fact thatsimply filling with xenon and never firing the laser also helpssubsequent fills.

Contrary to energy stability, the operating voltage is entirelyindependent of the previous history. This means that not a largepercentage of the xenon can be carried over to the next fill. There aredifferent ways how xenon could remain in the laser. Since xenon is avery heavy gas it may collect preferably on the bottom or in the MFTwhen the blower is not running. In that case, it should be removable bypumping the chamber with the leak checker to a pressure much lower thanwhat is available with the membrane pump. This still did not prevent thememory effect. Which would suggest that xenon gets trapped due to itslarge size in porous materials or virtual leaks in the chamber.

Weekend Xenon Test

FIG. 8I presents a weekend run in 2 kHz mode with a 30 ppm xenonmixture. No data are available without xenon, so this is merely astatement how well the chamber can perform. The total pressure did notincrease during the test. The linewidth increases for the first 2 hours,typical of heating of the fused silica LNP. Thereafter, the normal trendof decreasing linewidth for decreasing fluorine concentration isobserved. However, the linewidth continues to decrease and onlystabilizes after 3 injects.

The voltage continues to increase and also only stabilizes after 3injects. In conjunction with the linewidth data the voltage increase ismost likely not due to any impurities but simply because the mixturegets leaned out. Once the voltage is increased the injection intervalsshorten because the discharge is no longer blip-free.

During the test a fantastically low fluorine consumption rate wasobserved. Immediately after the refill, the laser was running for anincredible 15 hours and 28 million pulses without injection.

In summary, this test shows that with an addition of a small amount ofxenon to the gas mixture a KrF chamber can operate within specificationsfor 95% linewidth and dose stability. Very low fluorine consumption wasobserved.

Heavy Noble Gas Additives in ArF Laser

Applicants have conducted experiments with very small quantities of Krand Xe added to a typical ArF gas mixture. (A typical mixture is about0.08 to 0.12% fluorine, 3.5% argon and the rest neon.) Both Kr and Xesubstantially reduced the average 3 sigma of the laser. Without theadditives the 3 sigma for the laser was about 5%. About 6-10 ppm of Xereduced 3 sigma to about 4% (a 20% improvement). For the sameimprovement with Kr about 40 ppm were required.

As with the KrF laser the additives reduced the output of the laser. Forthe same discharge voltage pulse energy was reduced by about 1% for eachppm of Xe and about 0.2% for each ppm of Kr. Thus, an 8 ppm of xenon thepulse energy would be reduced by 8% and 40 pm of Kr would reduce theoutput by roughly the same amount about 8%.

Very Small Quantity Oxygen Addition

FIGS. 14 and 15 show the effect on the slug effect in a KrF laser ofadding minute quantities of oxygen to the laser gas. FIG. 14 shows adramatic reduction in the energy decrease occurring at about 22 to 35milliseconds into the burst. FIG. 15 shows that the 3-sigma variation isalso dramatically reduced with the addition of oxygen in the range ofabout 25 to 49 parts per million, but 25 ppm produces a reduction ofabout 10% in the pulse energy and 49 ppm produces a reduction of about20% Applicants have determined about 5 ppm provides significantimprovement in stability without significant detrimental effects.

Argon Fluoride Laser--Elimination of Gas Refill Syndrome with Oxygen

Applicants have discovered that the addition of oxygen also improvesperformance of very narrow band ArF lasers. Applicants have identifiedwhat they call a gas refill syndrome. They have discovered thatimmediately after replacing the laser gas in an ArF very narrow bandlaser, the laser performs very poorly in that the pulse energy issubstantially reduced. However, after setting overnight, the nextmorning the laser performs within specification.

This gas refill syndrome was eliminated with the addition of anextremely small quantity of oxygen such as about 2 to 3 parts permillion. Thus, the preferred laser gas mixture for the very narrow bandArF excimer laser is:

3.5 percent argon

0.1 percent fluorine

2-3 parts per million oxygen

remainder neon to 3 atmospheres.

Additional quantities of oxygen were added but the oxygen additionbeyond 5 ppm had no significant beneficial effect. Recommended ranges ofoxygen in both KrF and Arf lasers is between about 2 to about 7 ppm.Recommended ranges of Xe for KrF lasers is less than about 30 to 40 ppm.Recommended ranges of Kr for ArF lasers is less than about 40 ppm andrecommended Xe ranges is less than about 10 ppm.

Although this very narrow band laser has been described with referenceto particular embodiments, it is to be appreciated that variousadaptations and modifications may be made to the invention. AlthoughApplicants did not test radon in its lasers, they have concluded thatvery small quantities of radon gas would improve energy stabilitywithout substantial negative effects. Radon should be easier to ionizethan any other noble gas and it will not form long-lived compounds withfluorine. Therefore, it should like xenon in the KrF laser and kryptonin the ArF and KrF lasers aid preionization. Applicants expect that thebest concentration for radon would be similar to those discussed abovefor Xe and Kr. For example, sources of oxygen can be pure oxygen or anyof the oxygen referred to in U.S. Pat. No. 5,307,364. Also, the sourceof oxygen could be a solid such as aluminum oxide or potassium, whichcould be contained within the chamber environment and the oxygenemission, could be controlled with temperature. Therefore, the scope ofthe invention is to be limited only by the appended claims and theirlegal equivalent.

What is claimed is:
 1. An excimer laser comprisingA. a laser chambercomprised of fluorine compatible materials and containing:(1) twoelongated electrodes; (2) at least one preionizer; and (3) a laser gasdefining a total pressure and comprised of a first noble gas, fluorine,a buffer gas, and a stabilizing additive of less than 100 ppm; saidstabilizing additive being chosen from a group consisting of: oxygen atless than 10 ppm and a quantity of a second noble gas which is heavierthan said first noble gas.
 2. A laser as in claim 1 wherein said firstnoble gas is krypton and said stabilizing additive is xenon.
 3. A laseras in claim 1 wherein said first noble gas is krypton and saidstabilizer additive is radon.
 4. A laser as in claim 1 wherein saidfirst noble gas is argon and said stabilizing additive is krypton.
 5. Alaser as in claim 1 wherein said first noble gas is argon and saidstabilizing additive gas is xenon.
 6. A laser as in claim 1 wherein saidfirst noble gas is argon and said stabilizing additive gas is radon. 7.A laser as in claim 1 wherein said stabilizing gas is oxygen in aconcentration of between 2 and 7 parts per million.
 8. An excimer laseras in claim 1 wherein said excimer laser is a narrow band excimer laserand said fluorine has a partial pressure of less than 0.10 of the totalpressure.
 9. A narrow band excimer laser as in claim 8 and furthercomprising an output coupler having a reflectance of at least 25%.
 10. Anarrow band excimer laser as in claim 8 wherein said at least one prismis comprised of calcium fluoride.
 11. A narrow band excimer laser as inclaim 8 wherein at least one prism is three prisms, all comprised ofcalcium fluoride.
 12. A narrow band excimer laser as in claim 8 whereinthe partial pressure of fluorine is less than 0.06 percent of the totalgas pressure.
 13. A narrow band laser as in claim 8 wherein said excimerlaser is an ArF excimer laser and the concentration of oxygen is lessthan 5 ppm.